Optical fiber systems need reliable sources of short light pulses at high repetition rates to carry information through optical fibers. The information carried ranges from telephone conversations, to television signals, to digital computer data. Sources of short light pulses are also needed for lightwave instruments and generally in fields of scientific research. Currently available picosecond and sub-picosecond duration lasers require highly skilled personnel to operate the lasers because of the great number and complexity of optical adjustments and alignments. There is a need for an easily operable laser capable of producing sub-picosecond light pulses.
Short optical pulses are essential for testing the time response of fiber optic communication systems. The time response is related to the maximum rate of transmitting information. As the speed of telecommunications systems increases, the need for sub-picosecond testing impulse source becomes more acute.
Mode-locking is an important technique for generating ultrashort pulses. Mode-locking causes the oscillation energy of the laser to be condensed into a packet in time domain that travels back and forth inside a laser cavity--the consequences of fixing the phases of the longitudinal modes of a laser. It is well known that mode-locked lasers can produce short optical pulses on the order of sub-picosecond pulses or shorter. Mode-locking occurs spontaneously in some lasers if the optical path contains a saturable absorber (an absorber whose absorption decreases with increasing optical intensity). This is the method used to induce mode-locking in continuous dye lasers, due to the fact that a dye will absorb less power from a mode-locked train of pulses than from a random phase oscillation of many modes since the first form of oscillation leads to the highest possible peak intensities (for a given average power of the laser) and is attenuated less severely as a consequence. For a general discussion see Yariv, A., Quantum Electronics, 2nd Ed. New York: Wiley (1975); Optical Electronics, 3rd Ed. New York: Holt, Rinehart & Winston (1985).
Colliding pulse mode-locked (CPM) dye lasers have been reported to have produced light pulses of 1 picosecond or shorter. (See Fork et al., "Generation of optical pulses shorter than 0.1 psec by colliding pulse mode locking" Appl. Phys. Lett. vol. 38, No. 9, pp 671-2 (1961)). Ring lasers have the unique capability of oscillating simultaneously or independently in either of two distinct counter-propagating directions. Colliding pulse mode-locking (CPM) uses two synchronized counterpropagating pulses interacting in a thin saturable absorber to produce a short pulse.
Since becoming commercially available, erbium-doped fiber has become the preferred gain medium for generating short optical pulses in actively and passively mode-locked lasers. Mode-locking of erbium fiber lasers has been demonstrated using active modulators, saturable absorbers, polarization switching, and non-linear amplifying loop mirrors.
Saturation (the condition in which the population difference has reached a steady state) is very important to the laser theory. Saturation of the inverted population difference and hence the gain in an amplifying laser medium is what determines a laser's power output. When a laser oscillator begins to oscillate, the oscillation amplitude grows at first until the intensity inside the cavity is sufficient to saturate and therefore reduce the laser gain. Steady-state oscillation then occurs when the saturated laser gain becomes just equal to the total cavity losses, so that the net round trip gain is exactly unity. Gain saturation is thus the primary mechanism that determines the power level at which a laser will oscillate. Siegman, Lasers, University Science Books, p 207-8 (1986).
It is well known that semiconductors have two categories of properties: electronic and optical. (See Understanding Lasers, ch 8, IEEE, 1992.) For a fiber laser using a semiconductor as a saturable absorber, optical properties are important. The key parameters are bandgap and carrier lifetime. Bandgap is the energy spacing between the conduction band and the valence band and represents the energy (frequency) of light which just begins to produce electrons and holes in the semiconductor. Near the edge of the bandgap the number of available states for electron/hole production is limited. The word "gap" is used to describe the lack of energy levels for electrons and holes in between the conduction band and the valence band if the light frequency is too low.
In semiconductor material, electrons are in the valence bands under normal circumstances. When light with energy level equal to or greater than the semiconductor's band gap energy encounters the semiconductor, the light energy is absorbed, causing some electrons to transit to the conduction band and create electron-hole pairs. The electron-hole pairs eventually decay; the average lifetime is termed the "carrier lifetime."
There are two kinds of decay processes: radiative and non-radiative. In the radiative decay process, an electron drops back from the conduction band to valence band with a photon emitted at a frequency proportional to the band gap energy. The radiative decay process has a time constant of several nanosecond (10.sup.-9 S) which is intrinsic to the particular semiconductor material.
Non-radiative decay is a process by which an electron and a hole combine without generating light. Common mechanisms for non-radiative recombination are defects and impurities in a semiconductor. The time constant of non-radiative process depends on the density of the impurities and defects and can be shorter than 1 picosecond. Semiconductor saturable absorbers use the mechanism of absorption saturation. As mentioned previously, when a semiconductor absorbs light, the electrons transit from valence bands to conduction bands thus creating electron-hole pairs. When incident light intensity is high, there are so many transitions occurring that the valence band is almost empty and the conduction band is full, a semiconductor's ability to absorb light becomes weakened. This phenomena is known as the saturable absorber effect. The properties of a saturable absorber depend on its carrier lifetime. The shorter the carrier lifetime, the faster the decay of the electron-hole pairs, and the faster the recovery of the saturable absorber. The saturable absorber effect can be used to make a mode-locked laser. Due to loss reduction when light intensity is high, a laser with a saturable absorber inside tends to mode-lock to produce short pulses with high instantaneous intensity rather than continuous wave (CW) lasing of comparatively low intensity.
Recently, fiber lasers with semiconductor saturable absorbers have been reported. See Reddy et al., "A Turnkey 1.5 micrometer Picosecond Er/Yb Fiber laser", Optical Fiber Communication Conference, PD17, pp 71-4 (1993). Reddy et al. report a fiber laser employing erbium-doped fiber pumped by a Nd:YLF (1.053 um) microlaser pumped by a diode laser. Mode-locking was achieved using an InGAsP saturable absorber deposited on a InP substrate.
The Reddy et al. laser was claimed to provide self-starting mode-locking and to operate insensitive to polarization drift in the fiber. Therefore, the laser might be made to be a "turnkey" operation: that is to say, a laser which will operate at the turn of a key. However, several serious shortcomings still exist, leaving unmet commercial needs with respect to short pulse lasers.
The necessary semiconductor saturable absorber is not commercially available. In order to configure a mode-locking fiber laser, the semiconductor saturable absorber must have both the correct band gap and carrier lifetime. Although semiconductor saturable absorbers can be grown using MBE (molecular beam) or MOCVD (metal organic chemical vapor deposition), the carrier lifetime of the semiconductor material grown by these machines is usually too long to enable Er-doped fiber laser mode-locking.
Short carrier lifetime has been achieved using low temperature MBE or MOCVD technique, however the techniques are difficult to reproduce and to control. It is fair to say that the making of semiconductor saturable absorbers for mode-locked fiber lasers or other solid state lasers has been perceived as a "black art" of sorts! Thus, the need remains for a commercially available semiconductor saturable absorber to enable the commercial availability of turnkey mode-locking lasers.
Further needed is a commercially exploitable means of ion implantation. Ion implantation on semiconductor saturable absorbers is used to control the carrier lifetime of the electron-hole pairs. Without modifications, semiconductor materials such as InGaAsP/InP or GaAlAs/GaAs have natural carrier lifetimes of 1 to 8 nanosecond (a nanosecond equals 10-9 second). A carrier lifetime of this order is too long to produce the short optical pulse in an Er-doped fiber laser. One way to shorten the carrier lifetime in the semiconductor material is to use ion implantation. Ion implantation produces variations or "defects" in the semiconductor material. The defects serve as recombination centers for electron-hole pairs. By controlling the dosage and energy of ion implantation, the carrier lifetime can be reduced to the value required. The typical required value would be 1 to 100 picoseconds (a picosecond equals 10.sup.-12 second). Ion implantation is a better technique for producing defects in semiconductor saturable absorbers than the techniques of using low temperature MBE or MOCVD. Ion implantation allows the defect density and distribution to be well-controlled through varying the implantation dosage and energy. Moreover, low temperature growth of semiconductor saturable absorbers is an extremely difficult process. Most MBE or MOCVD machines are normally used to produce low defect material. Thus, low temperature operation for the creation of high defect material is "abnormal operation" of MBE or MOCVD machinery and, as such, requires extensive calibration.
Lamprecht et al. studied time-resolved,luminescence in proton bombarded InP (indium phosphate) in order to measure the lifetime of photoexcited carriers as a function of the damage dose. (See Lamprecht et al., "Ultrashort carrier lifetimes in H+Bombarded InP", Appl. Phys. Lett. 59 (8), pp 926-928, Aug. 19, 1991). Results were interpreted as meaning that the decrease in lifetime was a direct consequence of the defects of trapping and recombination centers produced by the bombardment. Photoexcited carriers in the proton bombarded InP demonstrated ultrashort lifetimes down to 95 femtoseconds.
Van der Ziel et al. reported using ion implantation to make a mode-locked semiconductor laser wherein the region of saturable absorption was introduced by proton bombardment. See "Generation of subpicosecond pulses from an actively mode-locked GaAs laser in an external ring cavity," Appl. Phys. Lett. 39(11), pp 867-869, Dec. 1, 1981. The treatment of inducing saturable absorption through proton bombardment converts initially stable emission from untreated samples (prior to bombardment) to a pulsating output from treated samples. Van der Ziel's external cavity consisted of a collimating lens and a reflecting mirror. Proton bombardment of the mirror facet introduced saturable loss, yielding sub-picosecond pulses in mode-locked semiconductor lasers.
Although a mode-locked semiconductor laser using ion implantation technique was demonstrated by van der Ziel et al., semiconductor lasers have a short gain recovery time (on the order of a nanosecond, which is 10-9 seconds). Mode-locking involves gain saturation by the semiconductor control medium and loss saturation by the saturable absorber.
There still remains a need for mode-locking lasers with much slower gain recovery times such as an Er-doped fiber amplifier which has gain recovery time of 10 millisecond (a millisecond being 10.sup.-3 seconds). Moreover, there still remains an unmet need for an easy to operate (i.e. self starting and polarization insensitive), compact laser capable of producing short optical pulses at high repetition rates.